1. Field of the Invention
The invention relates to a motor control device and an electric power steering system.
2. Description of the Related Art
Some of motor control devices used for an electric power steering system (EPS), or the like, output three-phase driving currents based on current values of the respective phases and a rotation angle (motor rotation angle) to a motor by executing current control using a triangular wave as a PWM carrier.
In such a motor control device, a resistor (shunt resistor) for detecting a current is provided in a power line corresponding to each phase. The current value of each phase is detected on the basis of a voltage between the terminals of each of these shunt resistors.
For example, as described in Japanese Patent Application Publication No. 2009-1055 (JP-A-2009-1055), in most cases, each shunt resistor is connected in series with a ground side of corresponding switching element pair (switching arm) that constitutes the driving circuit (PWM inverter). Then, each voltage between the terminals is amplified by a current detection circuit and is then input in an electronic circuit (microcomputer) that constitutes a current detector. Note that the configuration of the current detection circuit that amplifies and outputs the voltage between the terminals of the shunt resistor in this way is for example, described in Japanese Patent Application Publication No. 2007-295753 (JP-A-2007-295753). Then, the current value of each phase is detected on the basis of a difference between an output voltage (peak voltage) of the current detection circuit acquired (sampled) at a timing at which a PWM carrier that is a base of current control executed to drive the motor, that is, a triangular wave, reaches a “peak” and an output voltage (valley voltage) of the current detection circuit sampled at a timing at which the triangular wave reaches a “valley” immediately before the triangular wave reaches the “peak”.
That is, normally, at a timing at which the triangular wave reaches a “valley”, all the switching elements at the ground side (downstream side) that constitute the PWM inverter are turned off. That is, theoretically, each output voltage acquired at this timing is a ground voltage. In addition, all the switching elements at the ground side are turned on at a timing at which the triangular wave reaches a “peak”. Thus, by using a difference between these peak voltage and valley voltage, it is possible to accurately detect the current values of the respective phases by suppressing the influence of the switching noise.
In addition, some motors (brushless motors) that operate on the basis of the above-described three-phase driving currents include a resolver (motor resolver) as a rotation angle sensor. Then, when such a motor is set as a control target, the rotation angle is detected on the basis of two-phase output signals (a sinusoidal signal and a cosine signal) obtained by outputting exciting current to the motor resolver.
Note that the configuration of such a motor resolver and the details of a method of detecting a rotation angle on the basis of an output signal of the motor resolver are, for example, described in Japanese Patent Application Publication No. 11-160099 (JP-A-11-160099). Then, the configuration of an exciting circuit that outputs exciting current to a motor resolver is, for example, described in Japanese Patent Application Publication No. 5-52587 (JP-A-5-52587).
Incidentally, in recent years, with the improvement of manufacturing technique, a circuit through which a relatively large current flows may also be packaged. An increased number of the above-described motor control devices are also formed in such a manner that a current detection circuit used to detect the current of each phase and an exciting circuit for a motor resolver are formed in one package.
However, by forming the current detection circuit and the exciting circuit in one package in this way, these current detection circuit and exciting circuit share the grounding wire. This causes fluctuations in reference voltage (potential) in the amplified output of the voltage between the resistor terminals, amplified by the current detection circuit. Then, the fluctuations in reference voltage are incorporated into (superimposed onto) the output voltage of the current detection circuit as an excitation noise, resulting in a possible decrease in current detection accuracy.
The amplified output of the voltage between the resistor terminals, amplified by the current detection circuit, and detection of the current value of each phase based on the output voltage both are theoretically performed with reference to the ground voltage. However, actually, the grounding wire that grounds the current detection circuit also has a resistance (impedance). In addition, normally, a current output circuit, such as an exciting circuit and a driving circuit, is arranged at a location remote from a microcomputer that constitutes a current detector. Thus, the actual reference voltage of the current detection circuit is a voltage at a connecting point at which the current detection circuit and the exciting circuit that share the grounding wire are connected. Then, the voltage at the connecting point fluctuates on the basis of the impedance of the grounding wire with an exciting current output from the exciting circuit.
That is, as in the case of the microcomputer that constitutes the current detector, when the ground voltage is set as a reference, an excitation noise corresponding to fluctuations in the connecting point voltage used by the current detection circuit as a reference is superimposed on the output voltage of the current detection circuit. Therefore, as shown in FIG. 6, when the output voltages of the current detection circuit, acquired at sampling timings (L1, H1, L2, H2, . . . ) in one current detection process, are compared, the value of each peak voltage is offset by a variation (ΔV) of the excitation noise from the value of a corresponding one of the valley voltages, resulting in a possible decrease in current detection accuracy.